What do you mean with "Google lets it"? GrapheneOS is based on AOSP.

GrapheneOS only runs on the Google Pixels, and Google may decide to render future Pixels unusable for GrapheneOS (e.g. by preventing to unlock/relock the bootloader).

But another Android manufacturer could get to the point where GrapheneOS endorses them. It feels like it shouldn't be that hard for an Android manufacturer, and they would immediately get quite some attention. Maybe not mainstream attention, but largely profitable, I think.

GrapheneOS exists only because the Pixel's bootloader can be unlocked. Google could remove that option anytime, making it impossible to install GrapheneOS.